Conventional mobile communication systems provide voice communications, TV telephone communications and the like with stringent delay requirements by using circuit switching (CS) while providing data communications and the like with loose delay requirements by using packet switching (PS).
Meanwhile, when a new radio access technology (RAT) is introduced to a mobile communication system, the stability of the mobile communication system is given the top priority, and it is therefore general to use the new RAT to provide not various types of communication service but only limited types of communication service at an early stage of the introduction.
For example, assume that the LTE (Long Term Evolution) system is introduced as a new RAT to an area in which communication service is provided by using the UMTS (Universal Mobile Terrestrial System) as a RAT, the LTE system enabling communications which are more efficient and faster than those by the UMTS. In this case, considering that the LTE system is a mobile communication system using PS only, a conceivable introduction scenario is to provide communication service restricted to data communications at the early stage of the introduction, and to provide communication service, such as voice communications, with stringent delay requirements after the operation of the mobile communication system is stabilized (see Non-patent Document 1).
Next, referring to FIG. 12, an example of a method of providing communication service at an early stage of the introduction of the LTE system is described.
Here, a mobile station in an idle mode in the UMTS needs to perform the procedure shown in FIG. 12 in the UMTS, switch to the LTE system, and then start data communications in the LTE system, for example.
This is because the LTE system can provide more efficient and faster communications than the UMTS can. A concrete example of the method of providing communication service is described below with reference to FIG. 12.
As shown in FIG. 12, in Step 1, the mobile station transmits a “RRC Connection Request” for requesting a setup of a RRC connection, to a radio controller apparatus.
In Step 2, the radio controller apparatus transmits a “Radio Link Setup Request” for requesting a setup of a radio link, to a radio base station.
In Step 3, the radio base station performs a setup of the radio link in accordance with the “Radio Link Setup Request”, and then transmits a “Radio Link Setup Response” for reporting the completion of the setup of the radio link, to the radio controller apparatus.
In Step 4, the radio controller apparatus transmits a “RRC Connection Setup” for performing a setup of the RRC connection, to the mobile station.
In Step 5, the mobile station performs a setup of the RRC connection in accordance with the “RRC Connection Setup”, and then transmits a “RRC Connection Setup Complete” for reporting the completion of the setup of the RRC connection, to the radio controller apparatus.
In Step 6, the mobile station transmits a “Service Request” to a mobile exchange station through the radio controller apparatus.
In Step 7, the radio controller apparatus transmits the “Service Request” received from the mobile station, to the mobile exchange station.
In Step 8, the mobile exchange station transmits a “Security Mode Command” to the radio controller apparatus, so as to perform a security-related setup.
In Step 9, the radio controller apparatus transmits the “Security Mode Command” to the mobile station, so as to perform the security-related setup.
In Step 10, the mobile station transmits a “Security Mode Complete” for notifying a setup of a security-related parameter and the completion of the setup, to the radio controller apparatus.
In Step 11, the radio controller apparatus transmits the “Security Mode Complete” for notifying the completion of the setup of the security-related parameter, to the mobile exchange station.
In Step 12, the mobile station transmits an “Active PDP Context Request” for requesting a setup of a PDP context, to the mobile exchange station through the radio controller apparatus.
In Step 13, the radio controller apparatus transmits the “Active PDP Context Request” received from the mobile station, to the mobile exchange station.
In Step 14, the mobile exchange station transmits a “RAB Assignment Request” for requesting a setup of a radio access bearer (RAB), to the radio controller apparatus.
In Step 15, the radio controller apparatus determines whether or not to perform Inter-RAT handover to the LTE system, on the basis of information on the RAB whose setup is requested by the received “RAB Assignment Request”.
In Step 16, when determining to perform Inter-RAT handover to the LTE system, the radio controller apparatus transmits a “RAB Assignment Response” for reporting that the RAB whose setup is requested is not to be set up, to the mobile exchange station.
Moreover, when determining to perform Inter-RAT handover to the LTE system, in Step 17, the radio controller apparatus transmits a “Relocation Required” for requesting Inter-RAT handover to the LTE system, to the mobile exchange station.
In Step 18, the mobile exchange station transmits a “Relocation Command” for instructing Inter-RAT handover to the LTE system, to the radio controller apparatus.
In Step 19, the radio controller apparatus transmits a “Handover from UTRAN Command” for instruct to perform Inter-RAT handover to the LTE system, to the mobile station.
In Step 20, the mobile station changes the radio access technology of a target for connection to the LTE system, and then starts data communications in the LTE system.
By contrast, assume that a mobile station in an idle mode in the LTE system requests to start voice communications. In this case, since the LTE system does not provide any voice communication service bearer, the mobile station needs to be instructed to perform Inter-RAT handover to the UMTS at a stage for a setup of a voice service bearer in a procedure for a call setup in the LTE system, and to start voice communications in the UMTS.    Non-patent Document 1: 3GPP TR25.913 V7.3.0, “Technical specification group radio access network; Requirement for Evolved UTRA (E-UTRA) Release 7.5”
However, the above-described conventional mobile communication system has the following problems.
Specifically, in the conventional mobile communication system, if a RAT in which the mobile station is in an idle mode does not provide communication service requested by the mobile station, or if there is a different RAT capable of providing the communication service more efficiently than the RAT being used provides, the mobile station is instructed to perform handover to the different RAT in a quite late stage in a connection setup procedure (after reaching Step 18 in the example of FIG. 12). This increases processing load on each access node of the RATs, and increases connection delay occurring between a calling operation by a user and completion of establishing a User-plane (U-plane).